It is well known that random electrostatic surface charges on a moving web can give rise to coating non-uniformities for many types of coating systems, such as slide hopper bead or curtain coating. For example, U.S. Pat. No. 3,730,753, describes a coating non-uniformity introduced by random surface charges created on the web by a corona treatment process used to improve the adhesion of photographic emulsion to polyethylene-coated paper. Furthermore, U.S. Pat. No. 3,730,753 also describes an electrostatic charge neutralization system that can be used to remove the random surface charges, thereby eliminating the coating non-uniformity. Other examples of systems used to eliminate surface charge on a moving web prior to a liquid coating process may be found in U.S. Pat. Nos. 4,517,143 and 5,805,407. For each one of the charge neutralization systems described in these three patents, there is a set of one or more electrodes, connected to a high voltage power supply, used to deposit charge on a moving web while the web is wrapped around a backing roller which is typically connected to electrical ground. It should be noted that the efficiency and uniformity of the charge deposition process is highly dependent upon the web-to-roller capacitance. Variations in this capacitance, as caused by either intermittent air pockets between the web and backing roller, or by relief patterns in the surface of the backing roller, will create non-uniformities in the charge deposition process, giving rise to a non-uniform charge pattern on the web that can cause non-uniform coatings. No description is given in any of these patents regarding the relief patterns, if any, of the backing roller.
The moving web carries with it a boundary layer of air on the front side (the side to be coated) and the back side (the side facing the backing roller). For every conveyance system there exists a speed at which conveyance is limited by back surface air entrainment between the web and the conveying roller. If the surface of the backing roller is smooth and the moving web is conveyed around the roller, then an air film will arise between the web and roller, creating an air bearing between the two surfaces. This air film thickness (h) is a function of several parameters: 1) coating roller radius (R), 2) dynamic air viscosity (μ), 3) web speed (Uw), 4) roller speed (UR), and 5) web tension per unit width (T) and is given by the following equation:
                    h        =                  0.643          ⁢                                          ⁢                                    R              ⁡                              (                                                      6                    ⁢                                                                                  ⁢                                          μ                      ⁡                                              (                                                                              U                            w                                                    +                                                      U                            R                                                                          )                                                                              T                                )                                                    2              /              3                                                          Equation        ⁢                                  ⁢        1            [Knox & Sweeney, IECP J., V. 10, 1971].
For a given air viscosity and web tension, the air film thickness will increase with increasing web/roller speed and/or roller diameter. This increase in air film thickness results in decreased contact between the web and roller, with a concomitant loss in traction. If the speed is increased to the point that the air film thickness is of the same order, or larger than, the roughness of either the smooth roller surface or the surface of the web facing the roller, then traction will be lost completely, resulting in slippage of the roller against the web. This loss of traction can result in problems such as scratches or variations in tension and speed. In addition to loss of traction, the entrained air film thickness between the web and roller will result in a change in the web-to-roller capacitance, causing a non-uniform charge deposition and possibly leading to a non-uniform coating, as described previously.
It is known to provide means to remove or exhaust the boundary layers of air being carried on the back surface of a web and the surface of a roller when the two come into contact, increasing thereby the tractional contact of the web with the roller. Such means may include, for example, a pressure-loaded nip roller urged toward the conveying roller, the web passing therebetween. However, use of a nip roller may not be particularly desirable for several reasons including: 1) additional mechanical complexity to the apparatus that increases cost and reduces reliability, 2) increased potential for creasing of the web, particularly with thin webs, 3) possible marring of the surface of the web to be coated by the face-side nip roller, and 4) increased possibility of creating surface charges on either or both of the web surfaces due to the higher contact area produced by a nip roller, resulting in coating non-uniformities.
Such means may also include a relief pattern formed in the surface of the conveying roller into which the back-side boundary layer air may be exhausted from the web and escape. See U.S. Pat. No. 3,405,855 issued Oct. 15, 1968 to Daly et al., for example. In this patent, Daly et al. teach the use of a roller having circumferential venting grooves and supporting land areas to vent air carried by the underside of the traveling web, but there is no suggestion by Daly et al. that a grooved roller could be used at a charge neutralization station. Another example is provided by U.S. Pat. No. 4,426,757 issued Jan. 24, 1984 to Hourticolon, et al. In this patent, Hourticolon, et al. teach the manufacture and use of a roller having a surface relief consisting of a “finely branched network of compression chambers”, allowing the entrained air to be compressed into pockets rather than reducing the web traction. Both of these patents deal with purely conveyance roller issues and neither patent addresses the issue of web charge neutralization with such a roller surface pattern. It is not obvious that these roller surface patterns would perform well during a web charge neutralization process, because these relief patterns will create variations in the web-to-roller capacitance, causing charge non-uniformities that lead to coating non-uniformities.
U.S. Pat. No. 6,177,141 teaches the use of a relieved coating backing roller when using electrostatic assist to coat moving webs at high conveyance speeds. The use of a relief pattern that produces an electrostatic force variation at the coating point of less than a factor of about ten provides good traction at high speed and enables the use of electrostatic assist to provide high speed coatings while avoiding the gross failure of air entrainment at the coating point. However, it is not obvious that the use of a similar relieved backing roller would perform well if used in the charge neutralization process, because the non-uniformity in charge deposition associated with the use of such a relieved backing roller is not correlated with, nor can it be predicted by the electrostatic non-uniformity created by the same relieved backing roller when used as a coating backing roller.
It is known to provide a random pattern known as a mezzo pattern on a backing roller at a charge neutralization station. Such a pattern, known as a 50 mezzo coarse pattern, is shown in FIG. 1. This pattern is produced by chemically etching the surface of the roller, limiting the maximum practical depth achievable with this process. Typically, this maximum venting depth is in the range of 20 to 80 μm. Although the mezzo pattern does not produce objectionable patterns in a coated film, one problem encountered with this mezzo pattern is that at higher web conveyance speeds, effective contact with the web is lost.
There is a need therefore for an improved backing roller for use in a charge neutralization process that provides good conveyance and good charge uniformity at high speeds.